1. Field of the Invention
The present invention relates to a continuous annealing line for cold-rolled steel sheets and, more particularly, to a continuous annealing line having carburizing and nitriding furnaces which are disposed between a heating furnace or a soaking furnace and a cooling furnace and which continuously forms carbides, nitrides or carbonitrides on cold-rolled steel sheets.
2. Description of the Related Art
In recent years, the continuous annealing process has become popular to cope with demands for saving energy and remarkably shortening the process time.
It is well known that, in order to obtain a cold-rolled steel sheet having excellent press-formability, it is preferred to use a low-carbon steel or a ultra-low-carbon steel having a C content not higher than 0.01% and to add a carbide former such as Ti, Nb and Zr and a nitride former such as Al and B so as to form carbides and nitrides thereby to fix solid C and N dissolved in the steel when necessary.
Cold-rolled steel sheets produced by such a process, however, involve a problem in that, when such sheets are subjected to a zinc-phosphatizing process conducted as an under-coat treatment, the crystal grains of the film of phosphate become coarse or the precipitation of crystal grains of phosphate is locally failed with the result that the corrosion resistance after painting is reduced to such a level that can hardly be accepted when the sheets are intended for use on automobiles.
It has also been noticed that ultra-low-carbon steel tends to have a coarse structure in heat affected zone by welding, with the result that the strength in such zone becomes lower than that in the weld region or matrix. Thus, ultra-low-carbon steel is inferior to low-carbon Al killed steels in the aspects of strength and fatigue characteristic in welded portion.
Furthermore, ultra-low-carbon steel, which has a high ductility and, hence, large stickiness, tends to exhibit burrs in edges formed by shearing or punching when the shearing or punching is conducted under the same conditions as that for low-carbon Al killed steel. The burrs which have come off in the subsequent pressing step tend to cause flaws such as star-like defects. A demand therefore exists for improvement in punching characteristic of ultra-low-carbon steel.
Improvement in workability is essentially accompanied by a reduction in the amount of impurity elements to the surface region. Condensation of elements in the steel during annealing is reduced to lower the hardness at the surface of the steel sheet. Therefore, when such a steel sheet is worked by a press, defects tend to be generated in the surface of the steel sheet due to biting of the press die into the surface of the steel sheet and, in the worst case, the steel sheet may be cracked, unless the surfaces of the steel sheet are sufficiently lubricated.
As effective measures for obviating these problems, methods have been proposed in Japanese Patent Publication No. 1-42331 and in Japanese Patent Laid-Open Nos. 63-38556 and 2-133561 in which properties of surface regions of steel strips are changed by effecting carburization and nitriding on only the surface regions of the steel strips.
These literatures, however, do not at all show any equipment which continuously produces cold-rolled steel sheets, for press working, which contains dissolved C and N only in their surface regions.
Meanwhile, Japanese Patent Laid-Open No. 47-29230 discloses an apparatus for continuously carburizing or nitriding steel members. This apparatus, however, is intended to treat non-flat members and is not applied to continuous treatment of steel strips.
Japanese Patent Publication No. 55-26708, corresponding to U.S. Pat. No. 3,950,192, discloses a method for continuously carburizing a low-carbon steel strip. The continuous line used in this method has a pre-heating furnace, a carburizing furnace, a soaking furnace and a cooling furnace which are arranged in the mentioned order, in contrast to the present invention. Thus, in the method disclosed in Japanese Patent Publication No. 55-26708, the steel strip which has been carburized is heated in the soaking furnace at a temperature falling in austenitization temperature range, thereby causing carbon to be uniformly dispersed throughout the whole steel strip.
In order to effect a continuous carburizing on the surface of a steel sheet such that a desired amount of dissolved carbon exists only in the surface region of a predetermined depth from the surface, it is necessary that the steel sheet after annealing be carburized in a short time, e.g., within several tens of seconds, followed by quenching for preventing diffusion of carbon.
Practical carburization and nitriding of cold-rolled steel sheets in an industrial scale are most conveniently carried out by a carburizing/nitriding furnace which is equipped between a heating furnace and a cooling furnace in a continuous annealing line and maintained in a suitable temperature range. In this case, the velocity at which the steel sheet passes the continuous annealing line is determined by heat treatment which determines quality o the steel sheet itself. Therefore, carburizing/nitriding conditions are to be determined in accordance with the given annealing line velocity. The carburizing/nitriding conditions also have to be suitably changed in accordance with any change in the specifications of the steel sheet, such as material standards and dimensions. Furthermore, carburization and nitriding themselves have to adapt to different specifications of production.
In the carburization which is conducted in a short time, the reaction rate of solid-solution of carbon into steel is determined by the reaction on the surface of the steel sheet, so that a change in the carburization time, which is caused by any change in the velocity of passage of steel sheet in the continuous annealing line, significantly affects the concentration and depth of carburization.